In reply to Ashpara:
Varies a lot by manufacturer. For La Sportiva I generally go about three full Euro sizes down from my street shoe size for a snug-but-wearable sport clibming shoe; for Scarpa two sizes down is pushing the limit of wearability.
Rock 6 Run have a useful general sizing guide:
http://www.rockrun.com/wp/rock-shoe-sizing-guide/
And the guidelines per individual shoe model on the Banana Fingers website are also generally ok.